FIGS. 1A to 1C show an exemplary sequence of headers at the beginning of an Ethernet frame. An Ethernet frame generally includes a series of sub-frames nested hierarchically and corresponding to different layers of the Internet protocol suite.
FIG. 1A illustrates the first header of an Ethernet frame. The Ethernet protocol is a “data link layer” in the Internet protocol suite, such as Token Ring, PPP, HDLC, ATM, Wi-Fi, etc. The first six bytes B0 to B5 convey the destination MAC address, the next six bytes B6 to B11 convey the source MAC address, and bytes B12 and B13 (ETH TYPE) define the protocol of the “network layer” or “Internet layer” nested from byte B14 in the Ethernet frame, such as IP (IPv4, IPv6), ICMP, IPX, etc.
FIG. 1B illustrates a header of an IPv4 subframe that may start at byte B14. The header of an IPv4 frame includes various parameters, in particular the “protocol” identified in byte B24, and the source and destination IP addresses identified in eight bytes B26 to B33. The “protocol” byte defines the “transport layer” nested in the IPv4 frame from byte B37, such as TCP, UDP, or SCTP.
FIG. 1C illustrates a header of a TCP subframe that may start at byte B37. The four bytes B37 to B40 identify the source port and destination port.
The ETH TYPE parameter conveyed in bytes B12 to B13 of the Ethernet frame may also specify that the Ethernet frame contains several subframes of the same hierarchical level as the Ethernet frame, for example multiple Ethernet subframes. This is the case, for example, when the ETH TYPE parameter specifies the TRILL protocol (“Transparent Interconnection of Lots of Links”).
FIG. 2 illustrates the beginning of a TRILL frame nested from byte B14 in the Ethernet frame of FIG. 1A. A TRILL frame may convey multiple Ethernet sub-frames, each having a header of the same structure as FIG. 1A. Each Ethernet sub-frame is preceded by a preamble of six bytes, wherein the third and fourth bytes convey a parameter ERBN (“Egress RBridge Name”).
An Ethernet frame may in a so-called VLAN mode define virtual local networks. In this case, the ETH TYPE parameter is preceded by one to three four-byte labels used to identify the VLAN.
The IEEE 802.3ba standard provides an Ethernet connection that may, on four twisted-pair cables, reach a data rate of 40 Gbits/s, namely 10 Gbits/s per twisted-pair.
A network interface card capable of fully exploiting this data rate involves significant computing resources, in particular to decode the headers, calculate error checking signatures, extract the packets and direct the packets to their final destinations.